Polyethylene composition for blow molding having high stress cracking resistance

ABSTRACT

A polyethylene composition for producing blow-molded hollow articles, having the following features:
     1) density from greater than 0.952 to 0.957 g/cm 3 , determined according to ISO 1183-1 at 23° C.;   2) ratio MIF/MIP from 12 to 25;   3) MIF from 18 to 40 g/10 min.;   4) η 0.02  from 30,000 to 55,000 Pa·s;   5) long-chain branching index, LCBI, equal to or greater than 0.55;   6) ratio (η 0.02 /1000)/LCBI from 55 to 75.

FIELD OF THE INVENTION

In general, the present disclosure relates to the field of chemistry.More specifically, the present disclosure relates to polymer chemistry.In particular, the present disclosure relates to a polyethylenecomposition.

BACKGROUND OF THE INVENTION

In some instances, polyethylene compositions are useful for makingarticles by blow molding.

SUMMARY OF THE INVENTION

In a general embodiment, the disclosure provides a polyethylenecomposition having the following features:

-   1) density from greater than 0.952 to 0.957 g/cm³, alternatively    from 0.953 to 0.957 g/cm³, alternatively from 0.953 to 0.956 g/cm³,    determined according to ISO 1183 at 23° C.;-   2) ratio MIF/MIP from 12 to 25, alternatively from 15 to 22, where    MIF is the melt flow index at 190° C. with a load of 21.60 kg, and    MIP is the melt flow index at 190° C. with a load of 5 kg, both    determined according to ISO 1133-1;-   3) MIF from 18 to 40 g/10 min., alternatively from 20 to 40 g/10    min.;-   4) η_(0.02) from 30,000 to 55,000 Pa·s, alternatively from 36,000 to    55,000 Pa·s., alternatively from 36,000 to 50,000 Pa·s.; wherein    η_(0.02) is the complex shear viscosity at an angular frequency of    0.02 rad/s, measured with dynamic oscillatory shear in a plate-plate    rotational rheometer at a temperature of 190° C.;-   5) long-chain branching index, LCBI, equal to or greater than 0.55,    alternatively equal to or greater than 0.60, wherein LCBI is the    ratio of the measured mean-square radius of gyration R_(g), measured    by GPC-MALLS, to the mean-square radius of gyration for a linear PE    having the same molecular weight;-   6) ratio (η_(0.02)/1000)/LCBI, which is η_(0.02) divided by 1000 and    LCBI, from 55 to 75, alternatively from 55 to 70.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present disclosure will become betterunderstood with reference to the following description and appendedclaims, and accompanying drawing FIGURE where:

The FIGURE depicts a simplified process-flow of two serially connectedgas-phase reactors, which is useful in an embodiment of the ethylenepolymerization processes disclosed herein.

It should be understood that the various embodiments are not limited tothe arrangements and instrumentality shown in the drawing FIGURE.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the expression “polyethylene composition” embraces, asalternatives, both a single ethylene polymer and an ethylene polymercomposition. In some embodiments, the ethylene polymer composition ismade from or contains two or more ethylene polymer components. In someembodiments, the ethylene polymer components have different molecularweights. In some embodiments, the ethylene polymer composition is called“bimodal” or “multimodal” polymer.

In some embodiments, the present polyethylene composition is made fromor contains one or more ethylene copolymers.

In some embodiments, the addition of other components modifies one ormore of features 1) to 6).

The ratio MIF/MIP provides a rheological measure of molecular weightdistribution.

Another measure of the molecular weight distribution is provided by theratio M_(w)/M_(n), where M_(w) is the weight average molecular weightand M_(n) is the number average molecular weight, measured by GPC (GelPermeation Chromatography).

In some embodiments, M_(w)/M_(n) values for the polyethylene compositionrange from 15 to 30, alternatively from 15 to 25.

In some embodiments, M_(w) values are from 150,000 g/mol to 450,000g/mol, alternatively from 150,000 g/mol to 350,000 g/mol.

In some embodiments, M_(z) values, where M_(z) is the z-averagemolecular weight, measured by GPC, are equal to or higher 1,000,000g/mol, alternatively from 1,000,000 g/mol to 2,500,000 g/mol.

In some embodiments, ranges of LCBI values are:

-   -   from 0.55 to 0.85; alternatively    -   from 0.60 to 0.85; alternatively    -   from 0.55 to 0.70; alternatively    -   from 0.60 to 0.70.

In some embodiments, the polyethylene composition has at least one ofthe following additional features.

-   -   comonomer content equal to or less than 1.5% by weight,        alternatively from 0.1 to 1.5% by weight, with respect to the        total weight of the composition;    -   HMWcopo index from 0.5 to 5, alternatively from 0.5 to 3.

The HMWcopo index is determined according to the following formula:

HMWcopo=(η_(0.02) ×t _(maxDSC))/(10{circumflex over ( )}5)

where η_(0.02) is the complex viscosity of the melt in Pa·s, measured ata temperature of 190° C., in a parallel-plate (or plate-plate) rheometerunder dynamic oscillatory shear mode with an applied angular frequencyof 0.02 rad/s; the t_(maxDSC) is the time, in minutes, to reach themaximum value of heat flow (in mW) of crystallization (time at which themaximum crystallization rate is achieved, equivalent to the t1/2crystallization half-time) at a temperature of 124° C. under quiescentconditions, measured in isothermal mode in a differential scanningcalorimetry apparatus, DSC; and LCBI is the ratio of the measuredmean-square radius of gyration R_(g), measured by GPC-MALLS, to themean-square radius of gyration for a linear PE having the same molecularweight at a mol. weight of 1,000,000 g/mol.

In some embodiments, the comonomer or comonomers present in the ethylenecopolymers are selected from olefins having formula CH₂═CHR wherein R isan alkyl radical, linear or branched, having from 1 to 10 carbon atoms.

In some embodiments, the comonomer is selected from the group consistingof propylene, butene-1, pentene-1, 4-methylpentene-1, hexene-1, octene-1and decene-1. In some embodiments, the comonomer is hexene-1.

In some embodiments, the present composition is made from or contains:

-   A) 30-70% by weight, alternatively 40-60% by weight, of an ethylene    homopolymer or copolymer with density equal to or greater than 0.960    g/cm³ and melt flow index MIE at 190° C. with a load of 2.16 kg,    according to ISO 1133, of 40 g/10 min. or higher, alternatively 50    g/10 min. or higher;-   B) 30-70% by weight, alternatively 40-60% by weight, of an ethylene    copolymer having a MIE value lower than the MIE value of A),    alternatively lower than 0.5 g/10 min.

The above percent amounts are given with respect to the total weight ofA)+B). In some embodiments, component A) is an ethylene homopolymer.

In some embodiments, MIE ranges for component A) are.

-   -   40 to 150 g/10 min.; alternatively    -   50 to 150 g/10 min.; alternatively    -   40 to 100 g/10 min.; alternatively    -   50 to 100 g/10 min.

In some embodiments, the polyethylene composition is used for producingblow molded articles.

In some embodiments, the polyethylene composition is characterized bythe following properties.

-   -   Environmental stress crack resistance measured by FNCT 4        MPa/80° C. higher than 3 h;    -   Swell ratio higher than 170%;    -   Charpy aCN Impact (T=−30° C.) of 3 kJ/m² or higher; and    -   Absence of gels having gel diameter of higher than 700 μm.

In some embodiments, the blow-molding process is carried out by firstplasticizing the polyethylene composition in an extruder at temperaturesin the range from 180 to 250° C. and then extruding the polyethylenecomposition through a die into a blow mold, wherein the polyethylenecomposition is cooled.

In some embodiments, the polyethylene composition is prepared withoutlimitation as to polymerization process or catalyst. In someembodiments, the polyethylene composition is prepared by a gas phasepolymerization process in the presence of a Ziegler-Nana catalyst.

In some embodiments, the Ziegler-Natta catalyst is made from or containsthe product of the reaction of an organometallic compound of group 1, 2or 13 of the Periodic Table of elements with a transition metal compoundof groups 4 to 10 of the Periodic Table of Elements (new notation). Insome embodiments, the transition metal compound is selected amongcompounds of Ti, V, Zr, Cr and Hf. In some embodiments, the transitionmetal is supported on MgCl₂.

In some embodiments, the catalysts are made from or contain the productof the reaction of the organometallic compound of group 1, 2 or 13 ofthe Periodic Table of elements, with a solid catalyst component madefrom or containing a Ti compound supported on MgCl₂.

In some embodiments, the organometallic compounds are organo-Alcompounds.

In some embodiments, the present polyethylene composition is obtainableby using a Ziegler-Natta polymerization catalyst, alternatively aZiegler-Natta catalyst supported on MgCl₂, alternatively a Ziegler-Nattacatalyst made from or containing the product of reaction of:

-   a) a solid catalyst component made from or containing a Ti compound    and an electron donor compound ED supported on MgCl₂;-   b) an organo-Al compound; and optionally-   c) an external electron donor compound ED_(ext).

In some embodiments and in component a) the ED/Ti molar ratio rangesfrom 1.5 to 3.5 and the Mg/Ti molar ratio is higher than 5.5,alternatively from 6 to 80.

In some embodiments, the titanium compounds are tetrahalides orcompounds of formula TiX_(n)(OR¹)_(4-n), where 0≤n≤3, X is halogen,alternatively chlorine, and R¹ is C₁-C₁₀ hydrocarbon group. In someembodiments, the titanium compound is titanium tetrachloride.

In some embodiments, the ED compound is selected from alcohol, ketones,amines, amides, nitriles, alkoxysilanes, aliphatic ethers, and esters ofaliphatic carboxylic acids.

In some embodiments, the ED compound is selected among amides, estersand alkoxysilanes.

In some embodiments, the ED compounds are esters. In some embodiments,the esters are the alkyl esters of C1-C20 aliphatic carboxylic acids,alternatively C1-C8 alkyl esters of aliphatic mono carboxylic acids. Insome embodiments, the C1-C8 alkyl esters of aliphatic mono carboxylicacids are selected from the group consisting of ethylacetate, methylformiate, ethylformiate, methylacetate, propylacetate, i-propylacetate,n-butylacetate, and i-butylacetate. In some embodiments, the esters arealiphatic ethers, alternatively the C2-C20 aliphatic ethers,alternatively tetrahydrofuran (THF) or dioxane.

In the solid catalyst component the MgCl₂ is the basic support. In someembodiments, minor amount of additional carriers are used. In someembodiments, MgCl₂ is used as such or obtained from Mg compounds used asprecursors that are transformed into MgCl₂ by the reaction withhalogenating compounds. In some embodiments, MgCl₂ is used in activeform as a support for Ziegler-Natta catalysts. In some embodiments, theuse of these compounds in Ziegler-Natta catalysis is as described inU.S. Pat. Nos. 4,298,718 and 4,495,338. In some embodiments, themagnesium dihalides in active form used as support or co-support incomponents of catalysts for the polymerization of olefins arecharacterized by X-ray spectra in which the most intense diffractionline that appears in the ASTM-card reference of the spectrum of thenon-active halide is diminished in intensity and broadened. In someembodiments and in the X-ray spectra of magnesium dihalides in activeform, the most intense line is diminished in intensity and replaced by ahalo whose maximum intensity is displaced towards lower angles relativeto that of the most intense line.

In some embodiments, the solid catalyst component a) is obtained byfirst contacting the titanium compound with the MgCl₂, optionally in thepresence of an inert medium, thereby preparing an intermediate producta′) containing a titanium compound supported on MgCl₂. In someembodiments, the intermediate product a′) is prepared from the solidcatalyst component a) obtained by first contacting the titanium compoundwith a precursor Mg compound, optionally in the presence of an inertmedium. In some embodiments, intermediate product a′) is then contactedwith the ED compound which is added to the reaction mixture alone or ina mixture with other compounds, wherein the ED compound represents themain component, optionally in the presence of an inert medium.

As used herein, the term “main component” refers to the molar amountwith respect to the other possible compounds, excluding inert solventsor diluents used to handle the contact mixture. In some embodiments, theED treated product is subject to washings with the solvents to recoverthe final product. In some embodiments, the treatment with the EDcompound is repeated one or more times.

In some embodiments, a precursor of MgCl₂ is used as starting Mgcompound. In some embodiments, the precursor is a Mg compound of formulaMgR′₂ where the R′ groups can be independently C1-C20 hydrocarbon groupsoptionally substituted, OR groups, OCOR groups, chlorine, wherein R is aC1-C20 hydrocarbon groups optionally substituted, with the proviso thatthe R′ groups are not simultaneously chlorine. In some embodiments, theprecursors are the Lewis adducts between MgCl₂ and Lewis bases. In someembodiments, the precursors are MgCl₂ (R″OH)_(m) adducts wherein R″groups are C1-C20 hydrocarbon groups, alternatively C1-C10 alkyl groups,and m is from 0.1 to 6, alternatively from 0.5 to 3, alternatively from0.5 to 2. In some embodiments, the adducts are obtained by mixingalcohol and MgCl₂ in the presence of an inert hydrocarbon immisciblewith the adduct, operating under stirring conditions at the meltingtemperature of the adduct (100-130° C.). Then, the emulsion is quicklyquenched, thereby causing the solidification of the adduct in form ofspherical particles. In some embodiments, the spherical adducts areprepared as described in U.S. Pat. Nos. 4,469,648, 4,399,054, or PatentCooperation Treaty Publication No. WO98/44009. In some embodiments, thespherulization is the spray cooling method described in U.S. Pat. Nos.5,100,849 and 4,829,034.

In some embodiments, the adducts are MgCl₂.(EtOH)_(m) adducts wherein mis from 0.15 to 1.7 obtained subjecting the adducts with a higheralcohol content to a thermal dealcoholation process carried out innitrogen flow at temperatures between 50 and 150° C. until the alcoholcontent is reduced to the above value. In some embodiments, the processis as described in European Patent Application No. EP 395083.

In some embodiments, the dealcoholation is carried out chemically bycontacting the adduct with compounds capable to react with the alcoholgroups.

In some embodiments, these dealcoholated adducts are characterized by aporosity (measured by mercury method) due to pores with radius up to 0.1μm ranging from 0.15 to 2.5 cm³/g, alternatively from 0.25 to 1.5 cm³/g.

These adducts are reacted with the TiX_(n)(OR¹)_(4-n) compound (orpossibly mixtures thereof) mentioned above. In some embodiments, theadducts are reacted with titanium tetrachloride. In some embodiments,the reaction with the Ti compound is carried out by suspending theadduct in TiCl₄. In some embodiments, the TiCl₄ is cold. The mixture isheated up to temperatures ranging from 80-130° C. and maintained at thistemperature for 0.5-2 hours. In some embodiments, the treatment with thetitanium compound is carried out one or more times. In some embodiments,the treatment is repeated twice. In some embodiments, the treatment iscarried out in the presence of an electron donor compound. At the end ofthe process the solid is recovered by separation of the suspension. Insome embodiments, the separation is by settling and removing of theliquid, filtration, or centrifugation. In some embodiments, the solid issubject to washings with solvents. In some embodiments, the washings arecarried out with inert hydrocarbon liquids. In some embodiments, morepolar solvents are used. In some embodiments, the more polar solventshave a higher dielectric constant. In some embodiments, the polarsolvents are halogenated hydrocarbons.

The intermediate product is then brought into contact with the EDcompound under conditions able to fix an amount of the donor on thesolid. The amount of donor used is variable. In some embodiments, thedonor is used in molar ratio with respect to the Ti content in theintermediate product ranging from 0.5 to 20, alternatively from 1 to 10.In some embodiments, the contact is carried out in a liquid medium suchas a liquid hydrocarbon. In some embodiments, temperature at which thecontact takes place varies depending on the nature of the reagents. Insome embodiments, the temperature is in the range from −10° to 150° C.,alternatively from 0° to 120° C. Temperatures causing the decompositionor degradation of any specific reagents should be avoided even if thetemperatures fall within the range. In some embodiments, the time of thetreatment varies based upon other conditions such as nature of thereagents, temperature, and concentration. In some embodiments, thiscontact step lasts from 10 minutes to 10 hours, alternatively from 0.5to 5 hours. In some embodiments and to further increase the final donorcontent, this step is repeated one or more times. At the end of thisstep the solid is recovered by separation of the suspension. In someembodiments, the separation is by settling and removing of the liquid,filtration, or centrifugation. In some embodiments, the solid is subjectto washings with solvents. In some embodiments, the washings are carriedout with inert hydrocarbon liquids. In some embodiments, more polarsolvents are used. In some embodiments, the more polar solvents have ahigher dielectric constant. In some embodiments, the polar solvents arehalogenated or oxygenated hydrocarbons.

The solid catalyst component is converted into catalysts for thepolymerization of olefins by reacting the solid catalyst component withan organometallic compound of group 1, 2 or 13 of the Periodic Table ofelements, alternatively with an Al-alkyl compound.

In some embodiments, the alkyl-Al compound is selected from the groupconsisting of trialkyl aluminum compounds alkylaluminum halides,alkylaluminum hydrides and alkylaluminum sesquichlorides. In someembodiments, the trialkyl aluminum compounds are selected from the groupconsisting of triethylaluminum, triisobutylaluminum,tri-n-butylaluminum, tri-n-hexylaluminum, and tri-n-octylaluminum. Insome embodiments, the alkylaluminum sesquichlorides selected from thegroup consisting of AlEt₂Cl and Al₂Et₃Cl₃.

In some embodiments, the external electron donor compound ED_(ext)optionally used to prepare the Ziegler-Natta catalysts is the same as ordifferent from the ED used in the solid catalyst component a). In someembodiments, the external electron donor compound is selected from thegroup consisting of ethers, esters, amines, ketones, nitriles, silanesand their mixtures. In some embodiments, the external electron donorcompound is selected from the C2-C20 aliphatic ethers, alternativelycyclic ethers, alternatively cyclic ethers having 3-5 carbon atoms. Insome embodiments, the cyclic ether is selected from the group consistingof tetrahydrofuran and dioxane.

In some embodiments, the catalyst is prepolymerized by producing reducedamounts of polyolefin, alternatively polypropylene or polyethylene. Insome embodiments, the intermediate product a′) is subject toprepolymerization before adding the electron donor compound ED. In someembodiments, the solid catalyst component a) is subject toprepolymerization.

In some embodiments, the amount of prepolymer produced is up to 500 gper g of intermediate product a′) or of component a). Alternatively, theamount of prepolymer produced is from 0.5 to 20 g per g of intermediateproduct a′).

In some embodiments, the prepolymerization is carried out with the useof a cocatalyst such as organoaluminum compounds. In some embodiments,the cocatalyst is used in combination with an external electron donorcompound as discussed above.

In some embodiments, the prepolymerization is carried out attemperatures from 0 to 80° C., alternatively from 5 to 70° C., in theliquid or gas phase.

In some embodiments, the previously-described catalysts are used whenthe intermediate product a′) is subjected to prepolymerization.

In some embodiments, the process for preparing the polyethylenecomposition includes the following steps, in any mutual order:

-   a) polymerizing ethylene, optionally together with one or more    comonomers, in a gas-phase reactor in the presence of hydrogen; and-   b) copolymerizing ethylene with one or more comonomers in another    gas-phase reactor in the presence of an amount of hydrogen less than    step a);    wherein, in at least one of the gas-phase reactors, the growing    polymer particles flow upward through a first polymerization zone    (riser) under fast fluidization or transport conditions, leave the    riser and enter a second polymerization zone (downcomer) through    which the growing polymer particles flow downward under the action    of gravity, leave the downcomer and are reintroduced into the riser,    thereby establishing a circulation of polymer between the two    polymerization zones.

In the first polymerization zone (riser), fast fluidization conditionsare established by feeding a gas mixture made from or containing one ormore olefins (ethylene and comonomers) at a velocity higher than thetransport velocity of the polymer particles. In some embodiments, thevelocity of the gas mixture is between 0.5 and 15 m/s, alternativelybetween 0.8 and 5 m/s. As used herein, the terms “transport velocity”and “fast fluidization conditions” are as defined in “D. Geldart, GasFluidisation Technology, page 155 et seq., J. Wiley & Sons Ltd., 1986”.

In the second polymerization zone (downcomer), the polymer particlesflow under the action of gravity in a densified form, so that highvalues of density of the solid are reached (mass of polymer per volumeof reactor), which approach the bulk density of the polymer.

In other words, the polymer flows vertically down through the downcomerin a plug flow (packed flow mode), so that minor quantities of gas areentrained between the polymer particles.

In some embodiments, an ethylene polymer obtained from step a) has amolecular weight lower than the ethylene copolymer obtained from stepb).

In some embodiments, a copolymerization of ethylene to produce arelatively low molecular weight ethylene copolymer (step a) is performedupstream the copolymerization of ethylene to produce a relatively highmolecular weight ethylene copolymer (step b). In some embodiments and instep a), a gaseous mixture made from or containing ethylene, hydrogen,comonomer and an inert gas is fed to a first gas-phase reactor,alternatively a gas-phase fluidized bed reactor. The polymerization iscarried out in the presence of the previously described Ziegler-Nattacatalyst.

Hydrogen is fed in an amount depending on the specific catalyst used andto obtain in step a) an ethylene polymer with a melt flow index MIE of40 g/10 min. or higher. To obtain the above MIE range, in step a) thehydrogen/ethylene molar ratio is indicatively from 1 to 4, the amount ofethylene monomer being from 2 to 20% by volume, alternatively from 5 to15% by volume, based on the total volume of gas present in thepolymerization reactor. The remaining portion of the feeding mixture isrepresented by inert gases and one or more comonomers, if any. Todissipate the heat generated by the polymerization reaction, inert gasesare used and selected from nitrogen or saturated hydrocarbons,alternatively propane.

The operating temperature in the reactor of step a) is selected between50 and 120° C., alternatively between 65 and 100° C., while theoperating pressure is between 0.5 and 10 MPa, alternatively between 2.0and 3.5 MPa.

In some embodiments, the ethylene polymer obtained in step a) representsfrom 30 to 70% by weight of the total ethylene polymer produced in theoverall process, that is, in the first and second serially connectedreactors.

The ethylene polymer coming from step a) and the entrained gas are thenpassed through a solid/gas separation step to prevent the gaseousmixture coming from the first polymerization reactor from entering thereactor of step b) (second gas-phase polymerization reactor). In someembodiments, the gaseous mixture is recycled back to the firstpolymerization reactor while the separated ethylene polymer is fed tothe reactor of step b). A point of feeding of the polymer into thesecond reactor is on the connecting part between the downcomer and theriser, wherein the solid concentration is low and thereby avoiding anegative impact on the flow conditions.

The operating temperature in step b) is in the range of 65 to 95° C.,and the pressure is in the range of 1.5 to 4.0 MPa. In some embodiments,the second gas-phase reactor produces a high molecular weight ethylenecopolymer by copolymerizing ethylene with one or more comonomers. Insome embodiments and to broaden the molecular weight distribution of thefinal ethylene polymer, the reactor of step b) is operated byestablishing different conditions of monomers and hydrogen concentrationwithin the riser and the downcomer.

In some embodiments and in step b), the gas mixture entraining thepolymer particles and coming from the riser is partially or totallyprevented from entering the downcomer, thereby obtaining two differentgas composition zones. In some embodiments, a gas and/or a liquidmixture is fed into the downcomer through a line placed at a point ofthe downcomer, alternatively, the feeding point is in the upper partthereof. The gas and/or liquid mixture has a composition that differsfrom the gas mixture present in the riser. In some embodiments, the flowof the gas and/or liquid mixture is regulated so that an upward flow ofgas counter-current to the flow of the polymer particles is generatedand acts as a barrier to the gas mixture entrained among the polymerparticles coming from the riser. In some embodiments, the flow iscounter-current at the top of the riser. In some embodiments, a mixturewith low content of hydrogen is fed to produce the higher molecularweight polymer fraction in the downcomer. In some embodiments, one ormore comonomers are fed to the downcomer of step b), optionally togetherwith ethylene, propane or other inert gases.

The hydrogen/ethylene molar ratio in the downcomer of step b) is between0.005 and 0.2, the ethylene concentration is from 0.5 to 15%,alternatively 0.5-10%, by volume, the comonomer concentration is from0.05 to 1.5% by volume, based on the total volume of gas present in thedowncomer. The rest is propane or other inert gases. In someembodiments, because a low molar concentration of hydrogen is present inthe downcomer, a relatively high amount of comonomer is bond to the highmolecular weight polyethylene fraction.

The polymer particles coming from the downcomer are reintroduced in theriser of step b).

In some embodiments and because the polymer particles continue reactingand additional comonomer is not fed to the riser, the concentration ofthe comonomer drops to a range of 0.05 to 1.2% by volume, based on thetotal volume of gas present in the riser. The comonomer content iscontrolled to obtain the density of the final polyethylene. In the riserof step b) the hydrogen/ethylene molar ratio is in the range of 0.01 to0.5, the ethylene concentration being between 5 and 20% by volume basedon the total volume of gas present in the riser. The rest is propane orother inert gases.

In some embodiments, the polymerization process is as described inPatent Cooperation Treaty Publication No. WO2005019280.

EXAMPLES

The practice of the various embodiments, compositions and methods asprovided herein are disclosed below in the following examples. TheseExamples are illustrative and are not intended to limit the scope of theappended claims in any manner whatsoever.

The following analytical methods are used to characterize the polymercompositions.

Density

Determined according to ISO 1183-1 at 23° C.

Complex Shear Viscosity η_(0.02) (Eta (0.02))

Measured at angular frequency of 0.02 rad/s and 190° C. as follows.

Samples were melt-pressed for 4 min under 200° C. and 200 bar intoplates of 1 mm thickness. Disc specimens of a diameter of 25 mm werestamped and inserted in the rheometer, which was pre-heated at 190° C.The measurement was performed using a rotational rheometer. The AntonPaar MCR 300 was utilized, with a plate-plate geometry. Afrequency-sweep was performed (after 4 min of annealing the sample atthe measurement temperature) at T=190° C., under constantstrain-amplitude of 5%, measuring and analyzing the stress response ofthe material in the range of excitation frequencies ω from 628 to 0.02rad/s. The standardized basic software was utilized to calculate therheological properties, that is, the storage-modulus, G′, theloss-modulus, G″, the phase lag δ (=arctan(G″/G′)) and the complexviscosity, η*, as a function of the applied frequency, namelyη*(ω)=[G′(ω)²+G″(ω)²]^(1/2)/ω. The value of the latter at an appliedfrequency ω of 0.02 rad/s was the η_(0.02).

HMWcopo Index

To quantify the crystallization and processability potential of thepolymer, the HMWcopo (High Molecular Weight Copolymer) Index was used,which is defined by the following formula:

HMWcopo=(η_(0.02) ×t _(maxDSC))/(10{circumflex over ( )}5)

The t_(maxDSC) is determined using a Differential Scanning calorimetryapparatus, TA Instruments Q2000, under isothermal conditions at aconstant temperature of 124° C. 5-6 mg of sample were weighed andbrought into the aluminum DSC pans. The sample was heated with 20K/minup to 200° C. and cooled down also with 20K/min to the test temperatureto erase the thermal history. The isothermal test began immediatelyafter and the time was recorded until crystallization occurred. The timeinterval until the crystallization heat flow maximum (peak), t_(maxDSC),was determined using the vendor software (TA Instruments). Themeasurement was repeated 3× times and an average value was thencalculated (in min). If no crystallization was observed under theseconditions for more than 120 minutes, the value of t_(maxDSC)=120minutes was used for further calculations of the HMWcopo index.

The melt viscosity η_(0.02) value was multiplied by the t_(maxDSC) valueand the product was normalized by a factor of 100000 (10{circumflex over( )}5).

Molecular Weight Distribution Determination

The determination of the molar mass distributions and the means Mn, Mw,Mz and Mw/Mn derived therefrom was carried out by high-temperature gelpermeation chromatography using a method described in ISO 16014-1, -2,-4, issues of 2003. The specifics according to the mentioned ISOstandards were as follows: Solvent 1,2,4-trichlorobenzene (TCB),temperature of apparatus and solutions 135° C. and as concentrationdetector a PolymerChar (Valencia, Paterna 46980, Spain) IR-4 infrareddetector, capable for use with TCB. A WATERS Alliance 2000 equipped withthe following pre-column SHODEX UT-G and separation columns SHODEX UT806 M (3×) and SHODEX UT 807 (Showa Denko Europe GmbH, Konrad-Zuse-Platz4, 81829 Muenchen, Germany) connected in series was used.

The solvent was vacuum distilled under nitrogen and stabilized with0.025% by weight of 2,6-di-tert-butyl-4-methylphenol. The flowrate usedwas 1 ml/min, the injection was 500 μl and the polymer concentration wasin the range of 0.01%<conc.<0.05% w/w. The molecular weight calibrationwas established by using monodisperse polystyrene (PS) standards fromPolymer Laboratories (now Agilent Technologies, Herrenberger Str. 130,71034 Boeblingen, Germany)) in the range from 580 g/mol up to 11600000g/mol and additionally with hexadecane.

The calibration curve was then adapted to polyethylene (PE) by theUniversal Calibration method (Benoit H., Rempp P. and Grubisic Z., & inJ. Polymer Sci., Phys. Ed., 5, 753(1967)). The Mark-Houwing parameterswere for PS: k_(PS)=0.000121 dl/g, α_(PS)=0.706 and for PEk_(PE)=0.000406 dl/g, α_(PE)=0.725, valid in TCB at 135° C. Datarecording, calibration and calculation were carried out usingNTGPC_Control_V6.02.03 and NTGPC_V6.4.24 (hs GmbH, Hauptstraße 36,D-55437 Ober-Hilbersheim, Germany) respectively.

Melt Flow Index

Determined according to ISO 1133 at 190° C. with the specified load.

Long Chain Branching Index (LCBI)

The LCB index corresponds to the branching factor g′, measured for amolecular weight of 10⁶ g/mol. The branching factor g′, which allowsdetermining long-chain branches at high Mw, was measured by GelPermeation Chromatography (GPC) coupled with Multi-Angle Laser-LightScattering (MALLS). The radius of gyration for each fraction eluted fromthe GPC (with a flow-rate of 0.6 ml/min and a column packed with 30 μmparticles) was measured by analyzing the light scattering at thedifferent angles with the MALLS (detector Wyatt Dawn EOS, WyattTechnology, Santa Barbara, Calif.). A laser source of 120 mW ofwavelength 658 nm was used. The specific index of refraction was takenas 0.104 ml/g. Data evaluation was done with Wyatt ASTRA 4.7.3 andCORONA 1.4 software. The LCB Index was determined as follows:

The parameter g′ is the ratio of the measured mean square radius ofgyration to that of a linear polymer having the same molecular weight.Linear molecules show g′ of 1 while values less than 1 indicate thepresence of LCB. Values of g′ as a function of mol. weight, M, werecalculated from the equation:

g′(M)=<Rg ²>_(sample,M) /<Rg ²>_(linear ref.,M)

where <Rg²>, M is the root-mean-square radius of gyration for thefraction of mol. weight M.

The radius of gyration for each fraction eluted from the GPC (with aflow-rate of 0.6 ml/min and a column packed with 30 μm particles) wasmeasured by analyzing the light scattering at the different angles.Therefore, from this MALLS setup, mol. weight M and <Rg²>_(sample,M)were determined and g′ at a measured M=10⁶ g/mol was defined. The<Rg²>_(linear ref.,M) was calculated by the relation betweenradius-of-gyration and molecular weight for a linear polymer in solution(Zimm and Stockmayer WH 1949)) and confirmed by measuring a linear PEreference with the same apparatus and methodology described.

The same protocol is described in the following documents.

-   Zimm B H, Stockmayer W H (1949) The dimensions of chain molecules    containing branches and rings. J Chem Phys 17-   Rubinstein M., Colby R H. (2003), Polymer Physics, Oxford University    Press

Comonomer Content

The comonomer content was determined by IR in accordance with ASTM D6248 98, using an FT-IR spectrometer Tensor 27 from Bruker, calibratedwith a chemometric model for determining ethyl- or butyl-side-chains inPE for butene or hexene as comonomer, respectively. The result wascompared to the estimated comonomer content derived from themass-balance of the polymerization process.

Swell Ratio

The Swell-ratio of the studied polymers was measured utilizing acapillary rheometer, Göttfert Rheotester2000 and Rheograph25, at T=190°C., equipped with a commercial 30/2/2/20 die (total length 30 mm, Activelength=2 mm, diameter=2 mm, L/D=2/2 and 20° entrance angle) and anoptical device (laser-diod from Göttfert) for measuring the extrudedstrand thickness. The sample was melted in the capillary barrel at190°C. for 6 min and extruded with a piston velocity corresponding to aresulting shear-rate at the die of 1440 s⁻¹.

The extrudate was cut (by an automatic cutting device from Göttfert) ata distance of 150 mm from the die-exit, at the moment the piston reacheda position of 96 mm from the die-inlet. The extrudate diameter wasmeasured with the laser-diod at a distance of 78 mm from the die-exit,as a function of time. The maximum value corresponded to theD_(extrudate). The swell-ratio was determined from the calculation:

SR=(D _(extrudate) −D _(die))100%/D _(die)

where D_(die) was the corresponding diameter at the die exit, measuredwith the laser-diod.

Notched Tensile Impact Test AZK

The tensile-impact strength was determined using ISO 8256:2004 with type1 double notched specimens according to method A. The test specimens(4×10×80 mm) were cut from a compression molded sheet which was preparedaccording ISO 1872-2 (average cooling rate 15 K/min and high pressureduring cooling phase). The test specimens were notched on two sides witha 45° V-notch. Depth was 2±0.1 mm and curvature radius on notch dip was1.0±0.05 mm.

The free length between grips was 30±2 mm. Before measurement, testspecimens were conditioned at a constant temperature of −30° C. over aperiod of from 2 to 3 hours. The procedure for measurements of tensileimpact strength including energy correction following method A isdescribed in ISO 8256.

ESCR Belltest

Environmental Stress Crack Resistance (ESCR Bell Telephone Test) wasmeasured according to ASTM D1693:2013 (Method B) and DIN EN ISO22088-3:2006. 10 rectangular test specimens (38×13×2 mm) were cut from acompression-molded sheet, which has been prepared according to ISO1872-2 (average cooling rate 15 K/min and high pressure during coolingphase). The specimens were notched with a razor to a depth of 0.4 mmparallel to the longitudinal axes, centered on one of the broad faces.Afterward the specimens were bent in a U-shape with a bending device,with the notched side pointing upwards. Within 10 minutes from bending,the U-shaped specimens were put into a glass tube and filled with a 10%vol. aqueous solution of 4-Nonylphenyl-polyethylene glycol (ArkopalN100) at 50° C. and sealed with a rubber stopper. The specimens wereinspected visually for cracks hourly on the first day, then daily andafter 7 days on a weekly basis (on 168 h increments). The final valuewas the 50% failure point (F₅₀) of the 10 test specimen in the glasstube.

Environmental Stress Cracking Resistance According to Full Notch CreepTest (FNCT)

The environmental stress cracking resistance of polymer samples wasdetermined in accordance to international standard ISO 16770 (FNCT) inaqueous surfactant solution. From the polymer sample acompression-molded 10 mm thick sheet was prepared. The bars with squaredcross section (10×10×100 mm) were notched using a razor blade on foursides perpendicularly to the stress direction. A notching devicedescribed in M. Fleissner in Kunststoffe 77 (1987), pp. 45 was used forthe sharp notch with a depth of 1.6 mm.

The load applied was calculated from tensile force divided by theinitial ligament area. Ligament area was the remaining area=totalcross-section area of specimen minus the notch area. For FNCT specimen:10×10 mm²−4 times of trapezoid notch area=46.24 mm² (the remainingcross-section for the failure process/crack propagation). The testspecimen was loaded according to ISO 16770 with constant load of 4 MPaat 80° C. or of 6 MPa at 50° C. in a 2% (by weight) water solution ofnon-ionic surfactant ARKOPAL N100. Time until rupture of test specimenwas detected.

Charpy aCN

Fracture toughness determination by an internal method on test barsmeasuring 10×10×80 mm cut from a compression-molded sheet with athickness of 10 mm. Six of these test bars were notched in the centerusing a razor blade in the notching device mentioned above for FNCT. Thenotch depth was 1.6 mm. The measurement was carried out with the Charpymeasurement method under ISO 179-1, with modified test specimens andmodified impact geometry (distance between supports).

All test specimens were conditioned to the measurement temperature of−30° C. over a period of from 2 to 3 hours. A test specimen was thenplaced without delay onto the support of a pendulum impact tester inaccordance with ISO 179-1. The distance between the supports was 60 mm.The drop of the 2 J hammer was triggered, with the drop angle being setto 160°, the pendulum length to 225 mm and the impact velocity to 2.93m/s. The fracture toughness value was expressed in kJ/m² and given bythe quotient of the impact energy consumed and the initialcross-sectional area at the notch, aCN. Values for complete fracture andhinge fracture were used (see suggestion by ISO 179-1).

Cast Film Measurement

The Film measurement of gels was carried out on an OCS extruder type ME202008-V3 with 20 mm screw diameter and a screw length of 25 D with aslit die width of 150 mm. The cast line was equipped with a chill rolland winder (model OCS CR-9). The optical equipment consisted of a OSCfilm surface analyzer camera, model FTA-100 (flash camera system) with aresolution of 26 μm×26 μm. After purging the resin first for 1 hour tostabilize the extrusion conditions, inspection and value recording tookplace for 30 minutes afterwards. The resin was extruded at 220° C. witha take-off speed of ca. 2.7 m/min to generate a film with thickness 50μm. The chill roll temperature was 70° C.

The inspection with the surface analyzer camera provided the totalcontent of gels and the content of gels with diameter of higher than 700μm, as reported in Table 1.

Process Setup

The polymerization process was carried out under continuous conditionsin a plant including two serially connected gas-phase reactors, as shownin the FIGURE.

The polymerization catalyst was prepared as follows.

Procedure for the Preparation of the Catalyst Component

A magnesium chloride and alcohol adduct containing about 3 mols ofalcohol was prepared following the method described in example 2 of U.S.Pat. No. 4,399,054, but working at 2000 RPM instead of 10000 RPM. Theadduct was subjected to a thermal treatment, under nitrogen stream, overa temperature range of 50-150° C. until a weight content of 25% ofalcohol was reached.

Into a 2 L four-necked round flask, purged with nitrogen, 1 L of TiCl₄was introduced at 0° C. Then, at the same temperature, 70 g of aspherical MgCl₂/EtOH adduct containing 25% wt of ethanol were addedunder stirring. The temperature was raised to 140° C. in 2 h andmaintained for 120 minutes. Then, the stirring was discontinued; thesolid product was allowed to settle, and the supernatant liquid wassiphoned off. The solid residue was then washed once with heptane at 80°C. and five times with hexane at 25° C. and dried under vacuum at 30° C.

Into a 260 cm³ glass reactor provided with stirrer, 351.5 cm³ of hexaneat 20° C. and, under stirring, 7 g of the catalyst component wereintroduced at 20° C. Keeping constant the internal temperature, 5.6 cm³of tri-n-octylaluminum (TNOA) in hexane (about 370 g/l) and an amount ofcyclohexylmethyl-dimethoxysilane (CMMS) such as to have molar ratioTNOA/CMMS of 50, were slowly introduced into the reactor and thetemperature was brought to 10° C. After 10 minutes stirring, 10 g ofpropylene were carefully introduced into the reactor at the sametemperature during a time of 4 hours. The consumption of propylene inthe reactor was monitored and the polymerization was discontinued when atheoretical conversion of 1 g of polymer per g of catalyst was deemed tobe reached. The content was filtered and washed three times with hexaneat a temperature of 30° C. (50 g/l). After drying, the resultingpre-polymerized catalyst (A) was analyzed and found to contain 1.05 g ofpolypropylene per g of initial catalyst, 2.7% Ti, 8.94% Mg and 0.1% Al.

Internal Electron Donor Supportation on the Prepolymerized Catalyst

About 42 g of the solid prepolymerized catalyst were charged in a glassreactor purged with nitrogen and slurried with 0.8 L of hexane at 50° C.

Then, ethylacetate was carefully added dropwise (in 10 minutes) to havea molar ratio of 1.7 between Mg of the prepolymerized catalyst and theorganic Lewis base.

The slurry was kept under stirring for 2 h still having 50° C. asinternal temperature.

After that the stirring was stopped, the solid was allowed to settle. Asingle hexane wash was performed at room temperature before recoveringand drying the final catalyst.

Example 1

Polymerization

13.8 g/h of the solid catalyst with a molar feed ratio of electrondonor/Ti of 8, were fed using 5 kg/h of liquid propane to a firststirred precontacting vessel, into which also triisobutylaluminum(TIBA), diethylaluminumchloride (DEAC) and the electron donortetrahydrofuran (THF) were dosed. The weight ratio betweentriisobutylaluminum and diethylaluminumchloride was 7:1. The ratiobetween aluminum alkyls (TIBA+DEAC) to the solid catalyst was 5:1. Theweight ratio of alkyls to THF was 70. The first precontacting vessel waskept at 50° C. with an average residence time of 30 minutes. Thecatalyst suspension of the first precontacting vessel was continuouslytransferred to a second stirred precontacting vessel, which was operatedwith an average residence time of 30 minutes and kept also at 50° C. Thecatalyst suspension was then transferred continuously to fluidized-bedreactor (FBR) (1) via line (10).

In the first reactor ethylene was polymerized using H₂ as molecularweight regulator and in the presence of propane as inert diluent. 50kg/h of ethylene and 200 g/h of hydrogen were fed to the first reactorvia line 9. No comonomer was fed to the first reactor.

The polymerization was carried out at a temperature of 80° C. and at apressure of 2.9 MPa. The polymer obtained in the first reactor wasdiscontinuously discharged via line 11, separated from the gas into thegas/solid separator 12, and reintroduced into the second gas-phasereactor via line 14.

The polymer produced in the first reactor had a melt index MIE of about73 g/10 min and a density of 0.969 kg/dm³.

The second reactor was operated under polymerization conditions of about80° C., and a pressure of 2.5 MPa. The riser had an internal diameter of200 mm and a length of 19 m. The downcomer had a total length of 18 m,an upper part of 5 m with an internal diameter of 300 mm and a lowerpart of 13 m with an internal diameter of 150 mm. To broaden themolecular weight distribution of the final ethylene polymer, the secondreactor was operated by establishing different conditions of monomersand hydrogen concentration within the riser 32 and the downcomer 33. Thedifferent conditions were achieved by feeding 330 kg/h of a liquidstream (liquid barrier) via line 52 into the upper part of the downcomer33. The liquid stream had a composition different from that of the gasmixture present in the riser. The different concentrations of monomersand hydrogen within the riser, the downcomer of the second reactor andthe composition of the liquid barrier are indicated in Table 1. Theliquid stream of line 52 came from the condensation step in thecondenser 49, at working conditions of 49° C. and 2.5 MPa, wherein apart of the recycle stream was cooled and partially condensed. As shownin the FIGURE, a separating vessel and a pump were placed, in the order,downstream the condenser 49. The monomers to the downcomer were fed in 3positions (lines 46). In dosing point 1, located just below the barrier,14 kg/h of ethylene and 0.90 kg/h of 1-hexene were introduced. In dosingpoint 2, located 2.3 meters below dosing point 1, 3 kg/h of ethylenewere introduced. In dosing point 3, located 4 meters below dosing point2, 3 kg/h of ethylene were introduced. In each of the 3 dosing points, aliquid taken from stream 52 was additionally fed in ratio to ethylene of1:1. 5 kg/h of propane, 30.0 kg/h of ethylene and 30 g/h of hydrogenwere fed through line 45 into the recycling system.

The final polymer was discontinuously discharged via line 54.

The polymerization process in the second reactor produced high molecularweight polyethylene fractions. In Table 1 the properties of the finalproduct are specified.

The first reactor produced around 49% by weight (split wt %) of thetotal amount of the final polyethylene resin produced by both first andsecond reactors.

The comonomer (hexene-1) amount was about 0.8% by weight.

Example 2

Polymerization

13.7 g/h of the solid catalyst with a molar feed ratio of electrondonor/Ti of 8, were fed using 5 kg/h of liquid propane to a firststirred precontacting vessel, into which also triisobutylaluminum(TIBA), diethylaluminumchloride (DEAC) and the electron donortetrahydrofuran (THF) were dosed. The weight ratio betweentriisobutylaluminum and diethylaluminumchloride was 7:1. The ratiobetween aluminum alkyls (TIBA+DEAC) to the solid catalyst was 5:1. Theweight ratio of alkyls to THF was 70. The first precontacting vessel waskept at 50° C. with an average residence time of 30 minutes. Thecatalyst suspension of the first precontacting vessel was continuouslytransferred to a second stirred precontacting vessel, which was operatedwith an average residence time of 30 minutes and kept also at 50° C. Thecatalyst suspension was then transferred continuously to fluidized-bedreactor (FBR) (1) via line (10).

In the first reactor ethylene was polymerized using H₂ as molecularweight regulator and in the presence of propane as inert diluent. 50kg/h of ethylene and 200 g/h of hydrogen were fed to the first reactorvia line 9. No comonomer was fed to the first reactor.

The polymerization was carried out at a temperature of 80° C. and at apressure of 2.9 MPa. The polymer obtained in the first reactor wasdiscontinuously discharged via line 11, separated from the gas into thegas/solid separator 12, and reintroduced into the second gas-phasereactor via line 14.

The polymer produced in the first reactor had a melt index MIE of about77 g/10 min and a density of 0.969 kg/dm³.

The second reactor was operated under polymerization conditions of about80° C., and a pressure of 2.5 MPa. The riser had an internal diameter of200 mm and a length of 19 m. The downcomer had a total length of 18 m,an upper part of 5 m with an internal diameter of 300 mm and a lowerpart of 13 m with an internal diameter of 150 mm. To broaden themolecular weight distribution of the final ethylene polymer, the secondreactor was operated by establishing different conditions of monomersand hydrogen concentration within the riser 32 and the downcomer 33. Thedifferent conditions were achieved by feeding 330 kg/h of a liquidstream (liquid barrier) via line 52 into the upper part of the downcomer33. The liquid stream had a composition different from that of the gasmixture present in the riser. The different concentrations of monomersand hydrogen within the riser, the downcomer of the second reactor andthe composition of the liquid barrier are indicated in Table 1. Theliquid stream of line 52 came from the condensation step in thecondenser 49, at working conditions of 47° C. and 2.5 MPa, wherein apart of the recycle stream was cooled and partially condensed. As shownin the FIGURE, a separating vessel and a pump were placed, in the order,downstream the condenser 49. The monomers to the downcomer were fed in 3positions (lines 46). In dosing point 1, located just below the barrier,10 kg/h of ethylene and 0.50 kg/h of 1-hexene were introduced. In dosingpoint 2, located 2.3 meters below dosing point 1, 4 kg/h of ethylenewere introduced. In dosing point 3, located 4 meters below dosing point2, 4 kg/h of ethylene were introduced. In each of the 3 dosing points, aliquid taken from stream 52 was additionally fed in ratio to ethylene of1:1. 5 kg/h of propane, 32.5 kg/h of ethylene and 31 g/h of hydrogenwere fed through line 45 into the recycling system.

The final polymer was discontinuously discharged via line 54.

The polymerization process in the second reactor produced high molecularweight polyethylene fractions. In Table 1 the properties of the finalproduct are specified.

The first reactor produced around 49% by weight (split wt %) of thetotal amount of the final polyethylene resin produced by both first andsecond reactors.

The comonomer (hexene-1) amount was about 0.4% by weight.

Comparative Example 1

The polymer of this comparative example was a polyethylene compositionmade with a chromium catalyst in a loop reactor with <1% hexene ascomonomer, sold by Chevron Phillips with trademark HHM 5502.

Comparative Example 2

The polymer of this comparative example was a polyethylene compositionproduced in the Hostalen ACP slurry process with <0.5% butene ascomonomer, sold by LyondellBasell with trademark HS ACP 5831 D.

TABLE 1 Ex. 1 Ex. 2 Comp. 1 Comp. 2 Operative conditions first reactorH₂/C₂H₄ Molar ratio 2.5 2.6 — — C₂H₄ % 11.8 12.3 — — Density of A)(g/cm³) 0.969 0.969 MIE [2.16 kg] of A) (g/10 min.) 73 77 Split (wt. %)49 49 — — Operative conditions second reactor H₂/C₂H₄ Molar ratio riser0.31 0.31 — — C₂H₄ % riser 11.4 12.4 — — C₆H₁₂ % riser 0.33 0.23 — —H₂/C₂H₄ Molar ratio downcomer 0.09 0.11 — — C₂H₄ % downcomer 3.5 3.3 — —C₆H₁₂ % downcomer 0.49 0.32 — — H₂/C₂H₄ Molar ratio barrier 0.033 0.044— — C₂H₄ % barrier 6.7 7.3 — — C₆H₁₂ % barrier 0.62 0.43 — — FinalPolymer properties MIP [5 kg] (g/10 min.) 1.6 1.5 1.8 1.3 MIF [21.6 kg](g/10 min.) 30.9 30 31.2 21.2 MIF/MIP 19.6 19.9 17.5 16.4 Density(g/cm³) 0.9537 0.9557 0.9551 0.955 Swell ratio (%) 186 177 195 152 Mw(g/mol) 258400 240992 189960 231569 Mz (g/mol) 1546730 1253650 9414731106140 Mw/Mn 21.51 21.1 11.06 17.35 LCBI 0.67 0.63 0.9 0.68 Comonomercontent IR (% by weight) 0.8 0.4 0.8 (C₆H₁₂) <0.8 (C₄H₈) η_(0.02) 38,76841,224 57,813 35,667 (η_(0.02)/1000)/LCBI 57.9 65.4 64.2 52.5 AZK −30°C. (kJ/m²) 83.4 85.9 97.8 85.6 Charpy aCN, T = −30° C. (kJ/m²) 4.9 5.68.2 6.8 Belltest at 50° C. 888 378 11 118 FNCT* 4 MPa/80° C. (hours) 116 2.0 3.5 FNCT* 6 MPa/50° C. (hours) 77 37 7.5 21.5 Sum Gels/m² > 700 μm0.0 0.3 0.0 2.3 Sum Gels/m² total 423 738 7,121 12,599 HMW COPO Index1.4 0.9 — — Notes: C₂H₄ = ethylene; C₆H₁₂ = hexene; C₄H₈ = butene;*aqueous solution of 2% Arkopal N100

What is claimed is:
 1. A polyethylene composition having the followingfeatures: 1) density from greater than 0.952 to 0.957 g/cm³ determinedaccording to ISO 1183 at 23° C.; 2) ratio MIF/MIP from 12 to 25, whereMIF is the melt flow index at 190° C. with a load of 21.60 kg, and MIPis the melt flow index at 190° C. with a load of 5 kg, both determinedaccording to ISO 1133-1; 3) MIF from 18 to 40 g/10 min.; 4) η_(0.02)from 30,000 to 55,000 Pa·s; wherein η_(0.02) is the complex shearviscosity at an angular frequency of 0.02 rad/s, measured with dynamicoscillatory shear in a plate-plate rotational rheometer at a temperatureof 190° C.; 5) long-chain branching index, LCBI, equal to or greaterthan 0.55, wherein LCBI is the ratio of the measured mean-square radiusof gyration R_(g), measured by GPC-MALLS, to the mean-square radius ofgyration for a linear PE having the same molecular weight; and 6) ratio(η_(0.02)/1000)/LCBI, which is η_(0.02) divided by 1000 and LCBI, from55 to
 75. 2. The polyethylene composition of claim 1 comprising one ormore ethylene copolymers.
 3. The polyethylene composition of claim 1prepared with a Ziegler-Natta polymerization catalyst.
 4. Thepolyethylene composition of claim 3, wherein the Ziegler-Nattapolymerization catalyst comprises: the product of reaction of: a) asolid catalyst component comprising a Ti compound supported on MgCl₂,the component being obtained by contacting the titanium compound withthe MgCl₂, or a precursor Mg compound, optionally in the presence of aninert medium, thereby obtaining an intermediate product a′), thensubjecting a′) to prepolymerization and contact with an electron donorcompound; b) an organo-Al compound; and optionally c) an externalelectron donor compound.
 5. The polyethylene composition of claim 1,having at least one of the following additional features: comonomercontent equal to or less than 1.5% by weight, with respect to the totalweight of the composition; HMWcopo index from 0.5 to 5; wherein theHMWcopo index is determined according to the following formula:HMWcopo=(η_(0.02) ×t _(maxDSC))/(10{circumflex over ( )}5) whereinη_(0.02) is the complex viscosity of the melt in Pa·s, measured at atemperature of 190° C., in a parallel-plate rheometer under dynamicoscillatory shear mode with an applied angular frequency of 0.02 rad/s;tmaxDSC is the time, in minutes, to reach the maximum value of heat flow(in mW) of crystallization (time at which the maximum crystallizationrate is achieved, equivalent to the t1/2 crystallization half-time) at atemperature of 124° C. under quiescent conditions, measured inisothermal mode in a differential scanning calorimetry apparatus, DSC;and LCBI is the ratio of the measured mean-square radius of gyration Rg,measured by GPC-MALLS, to the mean-square radius of gyration for alinear PE having the same molecular weight at a mol. weight of 1,000,000g/mol.
 6. The polyethylene composition of claim 1, comprising: A) 30-70%by weight of an ethylene homopolymer or copolymer with density equal toor greater than 0.960 g/cm³ and melt flow index MIE at 190° C. with aload of 2.16 kg, according to ISO 1133, of 40 g/10 min. or higher; andB) 30-70% by weight of an ethylene copolymer having a MIE value lowerthan the MIE value of A).
 7. An article of manufacture comprising: thepolyethylene composition of claim
 1. 8. The article of manufactureaccording to claim 7, being a blow-molded articles.
 9. A process forpreparing the polyethylene composition of claim 1, comprising:polymerization steps carried out in the presence of a Ziegler-Nattapolymerization catalyst supported on MgCl₂.
 10. The process of claim 9,comprising the polymerization steps, in any mutual order: a)polymerizing ethylene, optionally together with one or more comonomers,in a gas-phase reactor in the presence of hydrogen; b) copolymerizingethylene with one or more comonomers in another gas-phase reactor in thepresence of an amount of hydrogen less than step a); wherein, in atleast one of the gas-phase reactors, the growing polymer particles flowupward through a first polymerization zone (riser) under fastfluidization or transport conditions, leave the riser and enter a secondpolymerization zone through which the growing polymer particles flowdownward under the action of gravity, leave the second polymerizationzone and are reintroduced into the first polymerization zone, therebyestablishing a circulation of polymer between the two polymerizationzones.